Lens Testing Glossary

Needed Adapters: Lenses come in a variety of mounts that aren't always related to cameras. Thankfully most lens mounts can be adapted to most cameras with the right gadgets. Even though you may see a reference to Nikon's f-mount in the reviews, most cameras out there can be similarly adapted. Just replace Nikon with your camera brand. eBay is a good place to find odd adapters.

Preferred Mounting: When a lens is mounted in the way it was originally intended, I will call that "normal" mounting. In order to work at their best at magnification above 1:1, many lenses need to be mounted in reverse. That will generally require that you find a reversing ring for your camera's lens mount (easy to find on eBay). The reversing ring will typically have a standard size lens thread on it - commonly 52 mm for Nikon. If you lens has 52 mm filter threads on it, the lens will screw right into the reversing ring, and you are ready to go. If your lens has a thread smaller than the reversing ring (say 49 mm) you will need a 49-52 mm step-up ring. If the lens has a larger thread (say 55 mm) you will need a 55-52 mm step-down ring. These are always available on eBay.

Working Distance: Working distance is the distance from the front of the lens to the object being photographed.

Magnification: Magnification is the ratio of the actual size of the object versus the size of the object when projected onto the detector. If the object measures 40 mm and when photographed it takes up 20 mm on the detector (half life size), the magnification is 1:2 or 0.50x. If the same 40 mm object is projected at twice life size on the detector (80 mm on the detector), the magnification is 2:1 or 2x.

Sharpness vs. Resolution: Sharpness and resolution are two different aspects of the same basic process - how objects are rendered by the lens and by how the lens and the detector work together.

Sharpness is how crisp and detailed an image looks when looking at the whole image in standard viewing conditions. This can be given an approximate number with lens testing (MTF50). The numbers on the sharpness graph tell you about how many sharp details you will get across the camera's detector. The higher the number the better.

Resolution is a little different. Resolution doesn't care if the detail is sharp, just that it can be seen. I like to think of resolution as how much detail you can see in an image when you are zooming up on a small part of it. Again, resolution can be approximated with a number (MTF10). It corresponds to the number of just resolved details you will see across the detector. Again, the higher the better.

The numbers that are provided in the testing are for unsharpened images. Using unsharpened images makes the number more consistent. Sharpening will increase the numbers by a variable amount, commonly somewhere around 30%. Sharpening affects the MTF50 more than the MTF10.

Nyquist Frequency: The Nyquist frequency is the resolution (or frequency) that aliasing begins. I won't go deeply into aliasing, but basically it is the point at which the lens outresolves the detector. With my testing it is the number of pixels across the detector measured vertically.

The MTF10 of a lens is very close to the resolution limit of the lens. Therefore if the MTF10 is larger than the Nyquist frequency, The lens is producing more information than the detector can handle. This can lead to image artifacts (moire patterns). This situation is much more common in normal imaging (not macro). In order to prevent these image artifacts, digital cameras will have an anti-aliasing filter in front of the detector.

Resolution vs Aperture: A larger (smaller number) aperture potentially allows for seeing finer details by limiting the amount of diffraction present in the image. Diffraction causes sharp edges to become progressively fizzier. Diffraction is increased when the aperture is closed down (higher number).

The problem is that lenses tend to produce fuzzier pictures as the aperture is opened - harder to construct and design lenses for larger apertures. This works opposite of diffraction. The end results is that most lenses take the highest-detailed pictures with the aperture closed down a notch or two.

Corner Fuzziness vs Aperture/Magnification: Lenses tend to produce the most detail in the center of the image. The loss of detail on the periphery tends to worsen as the aperture is opened. A measure of how well the lens is resolving details on the image periphery is to measure the sharpness in the center vs the sharpness on the periphery. The number I produce is how much less sharpness is seen on the edge vs. the center (in percent).

Edge sharpness tend to improve as the aperture is closed, but resolution tends to go down with this same action. There is a trade-off. With every lens there are trade-off in the design. You can't have everything at a reasonable price. A lens that has outstanding sharpness and resolution in the center and periphery of the lens is going to cost more than one that is only sharp in the center.

The size of the camera's detector also influences this performance. A smaller detector will only see the more central, better corrected portions of the image supplied by the lens. A larger detector will tend to perform a little worse when used with the same lens.

Corner Fuzziness

Rating

<10%

Outstanding

10-30%

Very good

30-50%

Good

50-70%

Visible

>70%

Pretty bad

Sharpness and Resolution vs Magnification: As the magnification is increased, the resolution and sharpness of the resulting image decreases. This is an inevitable result of the aperture moving farther away from the detector and is only slowed down by having a larger aperture on the lens (as long as the lens is made for such). Thankfully the resolution numbers go down slower than the magnification rises, so you tend to get a little more image detail as the magnification goes up (although there are diminishing returns as the magnification rises).

Adding more megapixels to the equation will tend to give more resolution and sharpness at lower magnification, but that advantage will tend to go away as the magnification rises.

Using a larger detector will also tend to give you a small advantage (at the same field of view).

Numbers that I would consider exceptionally good for MTF50 (unsharpened) would be (based on a D200):

Magnificiation

MTF50

1:4

1400

1:2

1300

1:1

1200

2:1

1100

3:1

1000

4:1

900

5:1

800

6:1

700

8:1

600

10:1

500

Performance Graphs: I have tried to make a rating system for sharpness and resolution that allows me to compare lenses against other lenses across the magnification range. The ratings vary from outstanding - about as good as you can find for this magnification - to poor - you can do quite a bit better at this magnification with a different lens.

The ratings don't necessarily imply that a poor lens performs poorly, just that you can do a lot better at this magnification with a different lens. If you don't plan on making giant pictures, a poor lens will make very good images.

My present graph is based on my D200 test camera. When the test camera is changed, the graph will also have to be changed. I have resisted upgrading my test camera because of this.

Resolving Power: This is a fairly simple calculation based on the resolution and the magnification. The MTF10 is measured in line widths/picture height - basically how many pixels across the height of the detector you can resolve. I tranlate that into picture width by multiplying it by 1.5. The width of the detector is 23.6 mm. That divided by the magnification id the width of the field of view. Take the width of the field o of view and divide it by the resolution across the width of the detector and you have a measurement of the smallest reoslvable detail.

The utility in listing this number is that you can see if the lens is getting more detail out of the image as the magnification rises. In most cases as the magnification rises, the resolving power also increases (although not as quickly as the magnification rises). This increase in resolving power will tend to peter out as the magnification rises. It will eventually hit a point where you get no significant increase in resolving power with an increase in magnification. That is called empty magnification. The is little use to increasing the magnification when empty magnification is in play - bigger picture, same detail.

Chromatic Aberration: Lenses don't naturally focus all of the colors in the same place (i.e. rainbows). Lenses must be carefully designed to make all of the colors focus in the right place. The designs are never perfect. More perfect = $$$. Color fringing tends to be better in the center of the image than in the periphery. This color fringing can be measured. I used to use a measurement in pixels, but have switched to a measurement that corrects the pixels for the distance from the center of the image (Ratings per Imatest website)

CA%

Severity

<0.04%

insignificant

0.04 - 0.08%

mild, difficult to see

0.08 - 0.15%

moderate, somewhat visible

>0.15%

strong, easily visible

Image Contrast: Image contrast is a measure of how black your blacks are and how white the whites are. Modern lens coatings tend to improve this quality.

Flare: Flare is caused by light moving through the lens in a path not designed for. Flare can be seen as spots or glare on the image when shooting into the sun. Modern lens coatings and lens design help to reduce flare. Flare can be a big problem with cheap uncoated lenses.

Distortion: Distortion is caused by unequal magnification as you move from the center to the edge of the image. This causes the image to look "sucked in" - pincushion distortion, or "bloated" - barrel distortion. Distortion is common in zoom lenses and very wide angle lenses. It is not all that common with macro and high-resolution lenses.

Stacking: Focus stacking is a method to increase the apparent depth of field in an image. This is accomplished by taking multiple images at various focus levels. There are software programs out there that take the sharp areas of each image and combine them into a single sharper image.

There is one freeware verison out there that is quite good called CombineZP. Other non-free programs are also out there that perform focus stacking, these include: Helicon Focus, Zerene Stacker, Photoshop